Corrosion, pitting, and tarnish resistant stainless steel



United States Patent CORROSION, PITTINd, Al ll) TARNISH RESISTANTSTAINLESS STEEL William A. Morgan and David E. C. King, Ottawa, 0n-

3,252,792 Patented May 24, 1966 ice In severely corrosive environments,free machining types, such as 416 and 430F, have lower resistance topitting corrosion than the corresponding unresulphurized types 410 and430, respectively.

tario, and Robert J. McClure, Bells Corners, Ontario, This problem ofpitting corrosion in the 400 Series of Canada, assignms, by direct 5meslle assignments, to stainless steels has long been recognized andadditions Her Majesty ,Queen inflght of Camila as of nickel ormolybdenum have been made to such steels salted byflle Mllnster of Mulesand g gf to improve the resistance thereof. Tarnishing or dis- No g fJuly 1964 colouration may occur as a result of any form of attack aims.(Cl. 75-126) 10 and is undesirable where an attractive surface 18required, This invention relates to stainless steels of martinsitic suchas caitrim for which 430 stainless steel is norand ferritic grades. Thisapplication is a continuationmally used. in-part of application SerialNo. 141,006, filed Septem- It is an object of this invention to providea chromium ber 27, 1961 (now abandoned). stainless steel having improvedresistance to tarnishing, Steels of the mentioned grades are frequentlyreferred pitting corrosition and overall corrosion consistent with to asthose of the 400 Series and are sometimes called low cost. plainchrominum steels. Thus, they contain chromium It ha been fou d that,when uranium is added as in excess of about 12% and substantially nonickel. Any an alloying constituent in chromium stainless steels,subnickel that may occasionally be used for alloying would stantiallyimproved overall and pitting corrosion renot be present in an amountthat would impart an 20 sistance and resistance to tarnishing isimparted to such austentic structure to the steel. steels. The uraniumis added in an amount of 0.001-

With chromium present in an amount of about or ex- 3.0% of the steel,depending on the levels of the other ceeding 12%, a continuous,impermeable, non-porous major elements present. film of hydrous oxideforms over the surface of the A large ranged of ferrous alloys wereprepared; comsteel under oxygenating (or passive) conditions. Thispositions of the main alloying elements ranging within film impartsconsiderable resistance to corrosion in an h E i shown i T m 1oxygenating medium, but breaks down in a corrosive medium whoseoxygenating power is low, such as a TABLE 1 solution containinghalogens, in which case the condi- Percent tions may be said to beactive. In completely active Carbon 0.03-0.8 conditions, stainless steelhas somewhat better overall Chromium 11.5- resistance to attack thanplain carbon steels but is rather Uranium 0.001,2.0 more prone topitting and grooving. Pitting takes pl Ni kel (whe present) Up to 2.5When active/passive cells are formed as a result of parts Molybdenum(when present) Up to 1,0 of the metal surface becoming exposed by filmremoval. 35 pi i is promoted in crevices i h t l f as Corrosion tests onsteels of such compositions have well as under foreign matter such asgrit, barnacles, and been carried out using an aerated solution of 10%the like. FeCl -6H O in 0.05 N HCl'. This solution was chosen TABLE 2Fitting corrosion table showing the efiect of a 4 hour exposure toacidified ferric chloride on weight loss per unit area and surfaceappearance Composition, percent Weight loss, Structure U/C Ratio grams/Surface Appearance sq. cm. Cr Ni 0 U Other Fcrritic 13. 5 0 0. 03 0 0 0.03380 Gratt'itular attack with mild pi mg. Do 13.6 i 0 0.03 0.22 7. 340. 03290 Granular attack only. Do 14. 0 0. 04 0 0 0. 03340 Deep granularattack. Do 14. 1 0. 04 0. 35 8. 76 0.03200 Fairly deep granular attack.

Surface brighter. Do 14. 0 0. 04 1. 89 22. 25 0. 00514 Surface bright.Pinhole attac Do 13.8 0. 04 1.43 35. 0.0116 Surtfacekbright. Pinhole a cMnrtensitic 11.8 1. 96 0.10 0 0 0. 0294 Surface tarnished. Heavy pinholeand granular attack. Do 11.7 1. 89 0. 09 0. 48 5. 33 0. 00857 Surfaceslightly tarnished.

Few pinholes. Do 11.6 1. 90 0.09 1. 29 14. 34 0. 00552 Surtfwe bright.Some pinhole a acir. Do 11. 5 1. 82 0. 11 1.87 17. 00 0. 00788 Veryslightly tarnished with granular attack. Ferritic 17.2 0. 10 0 0 0.02385 Fairly heavy granular attack with small pits. Do 0. 10 0. 0036 0.036 0. 01890 Surface somewhat tarnished with small pits. Do 0.10 0.0460. 46 0.01875 Surface less tarnished with fewer small pits. Do 0.10 0.272. 70 0. 00921 Surface bright with some pinholes only. Do 0. 20 0 00.02165 Suki-lace bright with some pin- 0 es. Do 24. 7 0. 20 0. 042 0.21 0.01130 Surface bright with fewer pinholes.

for its ability to attack the oxide film and to promote pitting. Testshave been carried out on a number of such steels in the as forgedcondition. Results of these tests are shown in Table 2. Themicrostructure of these steels therefore results from air cooling fromabout 1600. R, which, in chromium steels, would be equivalent toquenching from that temperature.

The improvement in corrosion resistant properties bears directrelationship to the ratio of uranium to carbon but this cannot beattributable to stabilization because of the fact that .the steels hadcooled quickly through the sensitization range.

Improved resistance to corrosion has also been obtained when alloys weretested in appropriate heat treated conditions.

If steels are used in operating conditions which involve the risk ofsensitization the presence of uranium in sufiicient quantity will alsostabilize them and thereby protect them against grain boundarycorrosion.

It is evident from the results quoted in Table 2 that uranium hasimproved the corrosion resistance, pitting resistance and tarnishingresistance, of a wide range of steels of ferritic and martensiticstructure, but more particularly the martensitic grades and the type 430ferritic steels (7 to 14 of Table 2). In the martensitic steels theaddition of 0.48 to 1.87% of uranium and in the type 430 steels theaddition of 0.0036 to 0.30% uranium shows a marked continuingimprovement in corrosion resistance and brightness. For any particularbase composition the corrosion resistance appears to vary directly asthe ratio of uranium to carbon. When the base alloy contains morechromium, or more nickel, the added corrosion resistance obtained isconsiderably enhanced by the presence of uranium so that optimum effectsare obtained with lower uranium to carbon ratios. When the base alloycontains more carbon, more uranium is required to maintain a givenuranium to carbon ratio, so that it is desirable to maintain the lowestlevels of carbon permissible in the various grades. However, in

view of the effect of high chromium in lowering the required optimumuranium to carbon ratio, alloys containing high carbon could be usedprovided they are accompanied by high chromium contents.

The concentration of the main elements Cr, C, U, and Ni and alloyingelements such as Mo (Where special structural modifications arerequired) should fall within the limits specified for the required gradeand Table 1. The concentrations of Mn, Si, S and P should fall withinthe ranges usually associated with chromium stainless steels, includingthe resulphurized grades 416 and 430 F.

We claim:

'1. A ferrous alloy of the chromium stainless steel type consistingessentially of carbon about 0.090.11%, chromium about 11.5-17.5 nickelwhen present up to about 2.5%, uranium about .00362.0%, and balanceiron.

2. A ferrous alloy of the chromium stainless steel type consistingessentially of carbon approximately 0.10%, chromium approximately 17%,uranium 0.00360.30%, and balance iron.

3. A ferrous alloy of the chromium stainless steel type consistingessentially of carbon approximately 0.10%,-

chromium approximately 14-18%, uranium 0.0036%, and balance iron.

4. A ferrous alloy of the chromium stainless steel type consistingessentially of carbon approximately 0.10%, chromium 14l8%, uranium0.001l.0% and balance iron.

No references cited.

DAVID L. RECK, Prmary Examiner.

P. WEINSTEIN, Assistant Examiner.

1. A FERROUS ALLOY OF THE CHROMIUM STAINLESS STEEL TYPE CONSISTINGESSENTIALLY OF CARBON ABOUT 0.09-0.11%, CHROMIUM ABOUT 11.5-17.5%,NICKEL WHEN PRESENT UP TO ABOUT 2.5%, URANIUM ABOUT .0036-2.0%, ANDBALANCE IRON.